I think we are as baffled as you since the windows firewall effectively doesn't block outbound connections, that may rule out the usual suspect that ashMaiSv.exe is being blocked by your firewall.
What kind of surprises me is you aren't getting any timeout issues as there will be no reply from your ISP email server. Is there any means in Incredimail to set a timeout delay as there is in OE, sorry I can be of no practical help as I have never used Incredimail. You could also enable the timeout in avast, Internet Mail, Customize, Advanced and set as the arrowed options in the image below.
Hopefully this should produce an error/notification pop-up with some information that may be useful.
Is there anything in common about the emails, attachments, large size, etc. that might cause the scanning to take a long time or does this hang occur for all emails, small size, no attachments, etc. ?
Can Incredimail turn off its bells an whistles and just send in text mode, if so try sending in text mode to see if that works. I know that kind of defeats the purpose of having Incredimail, but it will eliminate one possible interaction.
Do you still have another email program, if so testing that to see if it works.